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This undated product image provided by Mazda shows the 2015 Mazda6. Gas mileage for new cars and trucks in the U.S. averaged a record 24.1 miles per gallon last year, but the rate of improvement is slowing. Mazda led all ... more >

DETROIT (AP) - Gas mileage for new cars and trucks in the U.S. averaged a record 24.1 miles per gallon last year, but gains in fuel economy are slowing.

The Environmental Protection Agency says fuel economy last year rose one-half mile per gallon over the 2012 model year, mainly because automakers have improved gas engines and transmissions and added turbochargers to give smaller motors more power.

Although last year’s gain fell short of the 1.2 mpg improvement from 2011 to 2012, fuel economy is up almost 5 mpg since 2004, according to an annual report released by the agency on Wednesday. The EPA is predicting slower growth for this year, but officials still expect the industry to meet government standards that require the fleet to average 54.5 mpg by 2025.

The EPA, which bases its calculations on cars and trucks produced for sale in the U.S., is only predicting a 0.1 mpg increase from 2013 to 2014 as pickup truck and SUV production rises 2 percent.

Chris Grundler, head of the EPA’s office of transportation and air quality, also said Hyundai and Kia skewed the results because of delays in producing and selling higher-mileage new models in 2014. Without Hyundai and Kia, the improvement rises to 0.4 mpg, still short of the 2013 gain, according to the report.

But this year’s slower pace of improvement should reflect rising truck and SUV sales. Sales of light trucks are up 16.5 percent over last year, while car sales have risen only 2.3 percent, according to Autodata Corp. Trucks accounted for 49.3 percent of U.S. sales last year, but that has risen to 51.3 percent through September of this year.

The change is largely due to fairly steady gasoline prices through much of the year. Plus, trucks and SUVs boast improved gas mileage, although still well below that of cars. The EPA says the gas mileage of truck-based SUVs rose 27 percent from 2004 through last year, the biggest increase of any market segment. Cars and car-based SUVs each saw a 23 percent improvement.

The average cost of a gallon of regular unleaded gasoline nationwide was $3.27 on Wednesday, down 8 cents from a year ago, according to AAA. Gas is at its lowest price of the year, and the government predicts the average price for the year to be $3.45 a gallon, the lowest since 2011.

Grundler said the shift to trucks this year is not a surprise because people who need pickups for work have kept their old ones for a long time.

Still, he said, the auto industry is ahead of what the EPA expected at this point, and he expects improvements to vary from year to year depending on the new models that are introduced. “Some years there will be bigger redesigns than others,” he said. “Two years does not a trend make.”

For example, the aluminum-body Ford F-150 pickup, due out later this year, could raise the average mileage by itself because the F-150 is the top-selling vehicle in the nation, Grundler said. The truck’s reduction in mass is likely to yield significantly better mileage and reduced emissions over the current trucks, he said.

Mazda led all automakers with an average of 28.1 mpg. Honda was second at 27.4 mpg. Chrysler, General Motors and Ford were at the bottom of the rankings because they sell more pickups and SUVs.

The midsize Mazda 6, for example, already meets its fuel economy targets for 2019 mainly by reducing wind drag, and using lighter weight materials and a turbocharged engine, Grundler said.